1. Field of the Invention
The present invention relates generally to prevention of a degradation in transmission quality due to wavelength changes of an optical signal, and more particularly to a method, device, and system for controlling the wavelength of an optical signal.
2. Description of the Related Art
Widely known is an optical fiber communication system including an E/O converter for converting an input electrical signal into an optical signal, an optical fiber transmission line for transmitting the optical signal, and an O/E converter for converting the optical signal transmitted by the optical fiber transmission line into an output electrical signal. An optical amplifier is applied, so as to compensate for attenuation of the optical signal in the optical fiber transmission line.
In recent years, an erbium doped fiber amplifier (EDFA) operating in a wavelength band of 1.55 .mu.m which gives the smallest loss in a silica optical fiber has been put into practical use. In an optical fiber communication system to which an EDFA is applied, an amplified spontaneous emission (ASE) generated in the EDFA becomes a noise, so that an optical band-pass filter is generally used to remove the ASE.
A laser diode is frequently used as the E/O converter in the optical fiber communication system as mentioned above. The wavelength of an optical signal output from the laser diode changes because of a temperature change or aged deterioration of the laser diode. The pass band of an optical band-pass filter is set so that a desired optical signal passes the filter with a small loss (i.e., large transmittance). Accordingly, if the wavelength of an optical signal changes, the optical signal is attenuated or removed in the filter, causing a degradation in transmission quality or, in the worst case a reception incapability. Particularly in a system to which wavelength-division multiplexing (WDM) is applied, an optical band-pass filter in each wavelength channel has a narrow pass band, so that it is desirable to control the wavelength of an optical signal in each channel with a high accuracy.